The invention relates to the field of magnetic pickups for electric guitars, and more particularly, to an improved pole piece for an electric guitar having a configuration such that the magnetic field does not substantially interfere with the vibration of the string.
In the prior art, solid circular pole pieces have been used to direct the magnetic field from a permanent magnetic upward to the vicinity of a ferromagnetic guitar string. When the string is plucked, the vibrating string interacts with the magnetic field to cause changes in the paths of the flux lines. These changes occur at the same frequency or frequencies and intensity as the string vibrations. A coil wrapped around the pole piece generated a signal as the changing flux lines cut across the wires of the coil. This signal should have had all the frequency and intensity characteristics of the vibrating string if the string had been vibrating freely.
Unfortunately this was not the case. A freely vibrating guitar string has a very complex vibration pattern in the form of a precessing ellipse. The vibration has both primary and harmonic frequency components. Various characteristics of the vibration are important to good tonal quality in an electric guitar. Among the most important of these characteristics are: the length the vibrations lasts, i.e., the so-called "note sustain"; the richness (completeness) of the harmonic content reproduction; the accuracy of reproduction of the spectral content of the string vibration as the string is shortened or lengthened by pressing it against various frets on the fretboard; and, the reproduction of an "open" or "natural" sound.
The quality of the magnetic design of the pole piece in the guitar pickup has a great deal to do with whether or not these characteristics are achieved. If the pole piece is designed such that the guitar string passes through a significant portion of the magnetic field, the magnetic forces acting the guitar string will adversely alter the vibrational characteristics of the string. These forces dampens vibrations thereby reducing "note sustain". Further, the forces dampen certain harmonics more than others thereby altering the spectral content of the output signal. This adversely affects richness of the sound, the accuracy of note intonation and the "naturalness" of the sound.
Not only does the magnetic field dampen certain harmonics but it also alters the natural vibrational pattern of the string in a way that creates distortion in the output signal. The sonic effect can vary from sounding slightly harsh in minor cases to actually making the note sound out of tune in extreme cases.
One worker in the art attempted to solve this problem by making the pole piece a hollow tube made of ferromagnetic material. This design is detailed in U.S. Pat. No. 4,624,172 to McDougall. Unfortunately, the McDougall design did not completely solve the problem of string vibration dampening by the magnetic field of the pickup. Although the McDougall design did make some improvement over the solid pole pieces of the past, it has been discovered by the applicant that the McDougall design made surprisingly little improvement over the prior art.
Accordingly, a need exists for an improved pole piece design which does not substantially dampen string vibration.